1. Field of the Invention
The present invention relates to an LED (Light Emitting Diode) printer for forming a digital image by writing printing information on a photoconductor as an image carrier, with a light beam from an LED array and also relates to a copier, printer, facsimile apparatus or similar image forming apparatus including the LED printer.
2. Description of the Related Art
There is hitherto known an LED printer wherein a LED head group is constituted by a plurality of LED heads arrayed in the axial direction of a photoconductor as an image carrier of an image forming apparatus. An example of such an LED printer is disclosed in Japanese Unexamined Patent Application Publication No. 10-86438. There is also known an LED printer wherein a maximum photoconductive area width in the axial direction of a photoconductor can be subjected to exposure in divisions with a plurality of LED heads. Another example of such an LED printer is disclosed in Japanese Unexamined Patent Application Publication No. 2001-80118.
More specifically, in the LED printer disclosed in the above-cited Japanese Unexamined Patent Application Publication No. 10-86438, the LED head group is constituted by a plurality of LED heads arrayed in the axial direction of the photoconductor, and a maximum photoconductive area width in the axial direction of the photoconductor can be subjected to exposure in divisions with each of the LED heads. Furthermore, a plurality of LED heads may be arranged in a zigzag pattern in the axial direction of the photoconductor such that opposite end areas of adjacent exposable regions are overlapped with each other in the axial direction of the photoconductor. In addition, an odd number of three or more LED heads may be arranged in a zigzag pattern such that the number of the LED heads is larger on the upstream side in the rotating direction of the photoconductor than on the downstream side.
On the other hand, in the LED printer disclosed in the above-cited Japanese Unexamined Patent Application Publication No. 2001-80118, the maximum photoconductive area width in the axial direction of the photoconductor can be subjected to exposure in divisions with a plurality of LED heads, and the brightness of light is adjusted for each of the LED heads in an independent manner so as to eliminate unevenness in printing density caused by differences in brightness among the LED heads. Although no suggestion is made of the problem that unevenness in printing density occurs within the LED head among divided illumination areas thereof when an image is printed, it is proposed to eliminate unevenness in the intensity of light within an LED array unit by controlling the intensity of light for each of the divided illumination areas within the LED array unit.
Generally, an LED printer has high reliability because of employing no moving part, such as a polygon mirror used in a laser printer. Also, in the case of a printer requiring an output of a large-sized print, because the LED printer does not need an optical space for scanning a light beam in the direction of main scan, it is just required to dispose an LED head comprising an LED array and an optical element, e.g., a SELFOC lens, thus resulting in a reduction of the overall printer size. In recent years, therefore, LED printers have been widely used as ones substituted for laser printers.
In a laser printer, one light source with an output power of about 10 mW is energized to emit a laser beam that is scanned by a polygon mirror, an fθ lens, etc. On the other hand, in an LED printer, a plurality of LED's are arrayed in the direction of main scan in one-to-one relation to one pixel, and a current of about several to 10 mA is supplied to each of the LED's, thereby causing the LED to emit a light beam. Data transfer and illumination are controlled for each LED.
Accordingly, in a large-sized printer or copier, the number of driver IC's for driving the LED's increases and production yields reduce correspondingly. Another problem is that, because of necessity of improving the accuracy of parts to maintain the accuracy in array of write beams, the part price per piece is much higher in the large-sized printer or copier than in a small-sized printer or copier. For that reason, a printer is also proposed in which a plurality of LED array units each used in a small-sized inexpensive printer or copier are arranged in the direction of main scan so that a large-sized print can be outputted.
In the LED printer disclosed in the above-cited Japanese Unexamined Patent Application Publication No. 10-86438, two or three LED array units are arranged along the axis of the photoconductor and are subjected to exposure in divisions. The exposure in divisions is usually carried out, for example, by a dynamic method of illuminating LED's to make a scan in the direction of main scan on the dot-by-dot basis, or a static method of illuminating LED's all together or for each of several blocks at a time in the direction of main scan. That known LED printer employs the static method. Further, in that known LED printer, each LED array unit is divided into four parts for individual illuminations, and three LED array units are arranged to lie in one direction. As a whole, therefore, that known LED printer operates in a mode of illumination in 12 divisions.
Although there is no problem when an image is dark, a problem occurs in the case of a light image (low density) in that the image density changes across a boundary between the divided illumination areas. The reason is as follows. When the LED array unit is divided into plural areas, it is unavoidable that illumination areas at opposite ends are narrower in some actual use than inner illumination areas. This causes a difference in voltage drop attributable to, e.g., a difference in impedances of cables extended from a power supply, whereby larger currents are supplied to the illumination areas at the opposite ends and the intensity of light emitted from those illumination areas are relatively increased. In the case of a halftone image, therefore, the illumination areas at the opposite ends provide a higher image density, i.e., a darker image, than the inner illumination areas adjacent to the formers, thus resulting in unevenness in density. Further, a variation among the three LED array units depending on production lots also causes unevenness in the intensity of light in a halftone image.
In other words, unevenness in density occurs not only among the LED array units, but also among the divided illumination areas within one LED array unit. Hence, compensation of the intensity of light for each of the LED array units is not enough to satisfactorily deal with such a problem of unevenness in density.